Middle Marrat Research Articles

Multidisciplinary Integrated Approach Unlocks Fracture-Characterization Ambiguity

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 213771, “Unlocking Fracture-Characterization Ambiguity: A Multidisciplinary Integrated Approach,” by Ali Abughneej, Mohamed Al-Naqeeb, and Mohammed Al-Ostath, SPE, Kuwait Oil Company, et al. The paper has not been peer reviewed. _ Fracture systems play a significant role in production in the case of tight, low-porosity complex reservoirs. An exploration well in North Kuwait that targeted the Middle Marrat formation encountered uncertainty regarding reservoir compartments in an underexplored field, with minimal offset wells to properly evaluate its potential. A multidomain approach is proposed leveraging different types of expertise to tackle the petrophysical and geomechanical aspects of the fracture system governing the carbonate layer and assess its producibility and qualification for stimulation through hydraulic fracturing. Introduction A comprehensive study was performed on Well A targeting the Marrat formation, a tight, low-permeability carbonate Jurassic reservoir. During the last two decades, the operator has drilled many vertical and deviated wells through this formation. Hydrocarbon production from the closely spaced wells shows a major variation in flow rates, which has been attributed to the presence of uncharacterized fracture clusters as well as the limited understanding of reservoir compartmentalization. To address the challenges of production discrepancy, an innovative multidomain approach was proposed as an integrated solution in an attempt to understand the fracture systems governing the reservoir setting and its compartments and further assess their connectivity and extent to form a perspective on their possible contribution or hindrance to flow. An end-to-end comprehensive work flow was created combining the results of advanced formation evaluation, high-resolution borehole imaging, and state-of-the-art sonic imaging [the 3D far-field sonic (3DFFS) approach], followed by a fracture-stability analysis for the purposes of completion optimization and potential future hydraulic fracture design. Geological Setting Middle Marrat is targeted as the hydrocarbon-bearing zone, consisting of dolomitized oolitic grainstone to packstone facies, reflecting the high-energy inner-midramp shoal depositional environment.

Expression of the Toarcian Oceanic Anoxic Event in the mixed siliciclastic–carbonate system of the Arabian Plate: Insights from rare-earth element geochemistry and statistical analysis

Rare earth elements and yttrium (REEY) are considered powerful indicators of paleo-redox conditions. However, to date, the utilization of REEY remains limited to either pure carbonates or deep marine shales, with little evidence of attempts to test their ability to depict marine anoxia within mixed carbonate–siliciclastic systems. Across the Tethys Ocean, the Toarcian Oceanic Anoxic Event (T-OAE) is widely recorded by black marine shales with strong negative carbon isotopic excursion. Yet, the occurrence and expression of T-OAE across the mixed siliciclastic and carbonate system of the Arabian Plate has not been recorded or even explored. This provides a unique opportunity to test the robustness and viability of REEY, supported by statistical analysis, in revealing marine anoxia in a mixed system. Our results demonstrate a positive Ce anomaly within the middle Marrat, coupled with a positive Eu anomaly and aluminum-normalized Mn enrichment in the underlying lower Marrat carbonates. This could be hypothetically related to volcanic pulses, which triggered the T-OAE. Overall, the REEY geochemical record and detailed statistical analysis of this study indicate, for the first time, the possibility that T-OAE might have extended to the Arabian Plate for brief periods of time during the deposition of the middle Marrat shallow marine red beds.

Improving off-structure carbonate reservoir characterization using integrated Shared Earth Approach and Forward Stratigraphic Modelling: The Marrat Fm of southern Kuwait

A high-resolution reservoir characterization workflow is applied to assess the off-structure prospectivity of Lower Jurassic complex carbonate reservoirs of the Marrat Formation in the southern part of The State of Kuwait along the Dharif-Abduliyah fields. A detailed sedimentological and petrophysical assessment was integrated with 3D seismic interpretation and delineated vertical and lateral facies changes within the fields, which are affected by sets of complex fault patterns. Carbonate geo-bodies have been mapped using seismic stratigraphy, and 3D multi-seismic attribute and seismic acoustic inversion workflows. The Middle Marrat has been subdivided into 5 units (Middle Marrat 1-MM1 to Middle Marrat 5-MM5) that show vertical and lateral lithological and facies variability. The developed integrated sequence stratigraphic framework permitted to better assess the influence of accommodation variations as well as carbonate ecological settings on sedimentary facies development. This framework pointed to the influence of marine incursions and subsequent flooding surfaces, on the development of wackestone to mudstone textures interbedded within porous grainier inner ramp/shoal reservoir intervals. This integrated Shared Earth Approach that combined the various multi-disciplinary and multi-scale results was supported for the first-time by high resolution Forward Stratigraphic Modelling to provide a mapping and identification of petroleum systems elements including potential reservoir locations and baffles as well as stratigraphic trapping mechanisms. This workflow opens a new perspective to increasing hydrocarbon reserves along the Dharif and Abduliyah Fields as well as many other fields along the State of Kuwait.

Biostratigraphy and sequence stratigraphy of the Minjur and Marrat Formations (Upper Triassic to Middle Jurassic) in Kuwait

This paper summarizes sedimentological and biostratigraphic data for the Minjur and Marrat Formations in Kuwait and places them within an expanded sequence stratigraphic framework based on the letter and number scheme initiated by Sharland and others (2001). Only two sequences (Tr80 and J10) were identified in this interval in the original scheme, whereas, between Tr80 and the top of the Upper Marrat, 18 sequences are now recognized and their sequence boundaries and maximum flooding surfaces identified and illustrated herein. One new sequence (Tr90) was proposed in the underlying Jilh A Member. Three new sequences (Tr100, Tr105 and Tr110) are recognized in the Minjur Formation (= Lower Minjur in Saudi Arabia). Four sequences (J02, J04, J06 and J08) and part of a fifth (J10) are recognized in the Lower Marrat. Seven sequences (part of J10 and J11-J16) are recognized in the Middle Marrat. Three sequences (J17, J18 and J19) are recognized in the Upper Marrat. Biostratigraphic data are sparse for the Minjur and Marrat Formations. In the Marrat Formation, calcareous nannofossils are extremely rare, benthic foraminifers are relatively common but long-ranging and palynomorphs are sparse and most commonly non-age diagnostic. Published and unpublished proprietary data from 33 wells have been combined to determine the ages of the succession. Using palynology, the Minjur Lower Member was dated as Norian. An undifferentiated Norian-Rhaetian age range is assigned to the Middle and upper Members of the Minjur Formation. The Triassic/Jurassic boundary is placed just below the base of the Lower Member of the Marrat Formation, where cuttings samples from a thin interval of strata yielded long age-ranging palynomorphs. The Sinemurian/Pliensbachian boundary occurs in the Lower Marrat at the base of the S J06 transgressive systems tract. The Pliensbachian/Toarcian boundary, dated on the basis of Nannoceratopsis triceras and Lotharingius crucicentralis, is also in the Lower Marrat at the base of the S J10 transgressive systems tract. The contact of the Middle and Upper Marrat is unconformable and locally karsted but appears to correlate to the Toarcian/Aalenian boundary. The upper section of the Upper Marrat and the base of the overlying Dhruma Formation have not been cored but the Aalenian/Bajocian boundary is placed in the upper portion of Sequence J19. SB J20 is placed at the top of the Upper Marrat Member. Change in the age of the Late Triassic Baluti Formation in Iraq has resulted in changes in correlations of the Minjur and Marrat Formations to each other and to other formations in the region. The Lower Marrat is shown to be the shallow marine equivalent of the Upper Minjur siliciclastics in the subsurface Rub al Khali Basin and in the shallow subsurface Arabian Basin near the outcrop belt in Saudi Arabia. The Lower Marrat correlates to the Adaiyah Formation and the Upper Sarki Formation in Iraq. The Middle Marrat correlates to the Mus and Alan Formations in Iraq and to all of the Marrat Formation as defined in outcrop in Saudi Arabia. The Upper Marrat is largely represented by an Aalenian-age hiatus in the Saudi Arabian outcrop belt but a thin, condensed Aalenian section is present in the subsurface, basinward of the outcrop belt, and correlated with the Lower Dhruma (D1-D2). The Upper Marrat is correlated to the lower Sargelu Formation in Iraq. These new and revised correlations clarify the timing of the Late Triassic (Norian) uplift of the Qatar Arch.

Evidence for the early Toarcian Carbon Isotope Excursion (T-CIE) from the shallow marine siliciclastic red beds of Arabia

The Toarcian Oceanic Anoxic Event (T-OAE) and its corresponding Carbon Isotope Excursion (CIE) have been reported widely across the Tethyan region and globally. In Arabia, and based on ammonite dating, the time window of the T-OAE coincided with the deposition of the reddish siliciclastic unit of the Marrat Formation. However, no evidence of the T-OAE/CIE was ever reported from Arabia because these red beds were previously interpreted as continental deposits. Recently, these red beds have been recognized as shallow marine deposits which opened an opportunity to assess the occurrence and expression of T-OAE–CIE in Arabia. In this study, a multiproxy geochemical characterization was performed on the Toarcian Marrat Formation to infer the chemistry of the paleowater column and identify intervals of possible T-OAE/CIE in Arabia. While the low concentrations of redox-sensitive elements (Mo, U, V, Cr) may indicate a shallow oxic marine settings, the coupled negative δ13Corganic excursion and apparent increase in the chemical weathering suggests that the deposition of Marrat red beds coincided with the development of T-CIE and possibly time-equivalent to the T-OAE globally. The origin of reddening is interpreted to have occurred during the middle Marrat deposition due to the stabilization of unstable hydrous iron oxides to hematite under oxic marine conditions. The proposed model further indicates the possible development of source rocks in the deep, anoxic environment counterpart where the T-OAE may be expressed. Since our study documents the first record of the T-CIE and discuss the origin of shallow marine siliciclastic red beds in the Arabian Plate, this will have significant implications for the overall understanding of the T-CIE globally and for hydrocarbon exploration through realizations of potential new source rocks associated with the OAEs in the Toarcian and other time intervals.

Asymmetrical wave-dominated siliciclastic shorelines with evidence of along-strike variability of sedimentary processes: A revised interpretation for the Toarcian Marrat red beds, central Arabia

Marine siliciclastic red beds are common in the geological record; however, distinguishing them from continental red beds can be challenging without detailed sedimentological and stratigraphic analyses. Owing to the common tendency in the geological literature to relate the reddish color to the oxidization processes within the vadose zone, siliciclastic red beds of marine genesis can be misinterpreted as continental deposits. Unlike the other Toarcian successions in the Tethyan basins surrounding Arabia, the sedimentological and stratigraphic characteristics of the Marrat Formation of Arabia in general and its middle unit's red siliciclastic beds, in particular, have not been well constrained. For more than 50 years, the red siliciclastic beds of the Toarcian Marrat Formation have been interpreted as continental to marginal marine deposits based on their reddish color and the absence of fossils. The lack of supporting sedimentological and stratigraphic evidence to this interpretation raises critical concerns regarding its validity.Along the Jurassic outcrop belt in central Arabia, three outcrop localities of the middle Marrat reddish siliciclastic deposits (with a total thickness of more than 120 m) demonstrate sedimentological and ichnological features that are contrary to the non-marine to marginal marine interpretation. The integration between sedimentology and ichnology reveals the existence of asymmetrical wave-dominated shorelines with evidence of river hyperpycnal flows and tidal processes during the middle Marrat time. A total of seventeen facies are identified and grouped into four facies associations: mixed wave and river-dominated delta front to prodelta mudstone (FA1), shoreface (strandplain) and underlying shelf mudstone (FA2), tidally-influenced distributary channel (FA3), and offshore carbonates (FA4). Stratigraphically, six major parasequences (each with a thickness of ~ 3–13 m) were mapped and correlated between the three measured sections with clear overall progradational to retrogradational stacking patterns.To the best of our knowledge, this study is the first to document the presence of Toarcian shallow-marine siliciclastic red beds on the Arabian Plate. It displays an example of a case study where ichnology and process-based sedimentology were integrated to constrain the depositional environment of these red beds. This study provides new insights into the depositional settings of the Toarcian red siliciclastic beds of the Marrat Formation, with implications to the stratigraphy of Arabia and the Early Jurassic paleogeography of the western Tethys and, in particular, the Arabian Plate.

Successful Recovery and Stimulation in a Long HP/HT Horizontal Well in One Intervention

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 188567, “Successful Recovery and Stimulation of a Long HP/HT Horizontal Well in One Intervention: A Case History of Acid Fracturing and New Generation of Diversion Systems Combination, Middle Marrat Carbonate Formation,” by Leopoldo Sierra, Alaa Eldine Alboueshi, Mohamed Elmofti, Walid Eid, Salma Sadeddin, and Ahmed Allam, Halliburton, and Mohamed Al Othman, Zamzam Ahmed, Erkan Fidan, Ibrahim Al-Zaidani, Neoq Nilotpaul, Meshari Ashkanani, Ali Buhamad, Mohammed Abdullah Al-Dousari, Abdul-Samad Mohammed Ahmed, and Yousef Al-Matrouk, Kuwait Oil Company, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed. The case history presented in the complete paper describes the performance of an acid-fracturing intervention in a high-pressure/high-temperature (HP/HT) well in which this intervention was the last procedure considered to evaluate the productivity of a Marrat Formation well. The post-acid-fractured well productivity index (PI) showed the high quality of the stimulation performed in a challenging environment, demonstrating the effectiveness of the new diversion system for creating selective fractures in a horizontal wellbore with multiple perforation clusters. Introduction Completion of deep HP/HT carbonate formations in northern Kuwait in overpressured environments comprising interbedded carbonate bodies, complex tectonics, and stress variations resulting in strike/slip or reserve fault regimes is challenging for operator and service companies. Because of the low-permeability conditions, it is necessary to generate long and conductive fractures to produce the reservoir effectively. Additionally, this aggressive fracture- stimulation requirement needs to overcome the high-working-pressure limitations of the completion, in addition to the nature and corrosiveness of the produced hydrocarbon, which makes se-lection of the proper stimulation reactive-fluid system important. These challenges are even more crucial in cases having drilling and completions issues. The alternative implemented for the well candidate presented in the complete paper considers the completion and stimulation of a well having an existing long perforated interval, where the implementation of a selective fracture stimulation of a highly deviated wellbore (88° or almost horizontal) was challenging. Geology, Reservoir, and Geomechanics of Middle Marrat Formation The Marrat reservoir is subdivided into three lithostratigraphic units: Lower, Middle, and Upper. The well candidate was drilled and completed to produce hydrocarbon reserves from the Middle Marrat Formation. The Middle Marrat unit represents a wide range of rock types related to high-stand precursor facies in a carbonate setting, which is overprinted by diagenetic alteration in refluxed and recrystallized dolomites. Eight gross flow zones have been identified bounded by major sequence-stratigraphic surfaces that frame the background of genetically correlated units. Reservoir quality and vertical connectivity can be controlled further by placing properly distributed hydraulic fractures, vertically and laterally, to connect the prospective reservoir and existing natural-fracture networks to improve hydrocarbon recovery.

Challenges in Testing and Completion of an HP/HT Reservoir in Kuwait

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 167647, ’Challenges in Testing and Completion of Highly Sour HP/HT Reservoirs in the State of Kuwait,’ by Y. Al-Salali, H. Al-Bader, V. Duggirala, M. Ayyavoo, P. Subban, A.R. Al-Ibrahim, and A. Rajkhowa, Kuwait Oil Company, prepared for the 2013 SPE Kuwait Oil and Gas Show and Conference, Mishref, Kuwait, 7-10 October. The paper has not been peer reviewed. Recently, a new structure located in northwestern Kuwait was drilled and tested with a deep-drilling rig. Formations in this deep structure have very low porosity and permeability. During short-term testing, wells produced oil and gas with, unexpectedly, the highest hydrogen sulfide (H2S) content of any reservoir so far discovered in Kuwait. Because the reservoir was rated as high-pressure/ high-temperature (HP/HT) and sour, it was a challenge to test the wells safely and successfully. Introduction Jurassic carbonate reservoirs in Kuwait fields are deep, low-permeability, and HP/HT and generally contain sour volatile oil/condensate and gas. Formations containing these deep reservoirs are tight and naturally fractured carbonates. Najmah (NJ), Sargelu (SR), Upper Marrat, Middle Marrat, and Lower Marrat are the five formations proved to hold hydrocarbon- bearing reservoirs. NJ and SR formations are tighter than the three Marrat formations. The NJ formation consists of limestone at the upper part and shale at the bottom. The SR formation consists mainly of shale and limestone. Both are naturally fractured reservoirs, so fractures play an important role in production. The Marrat formations consist mainly of limestone, occasionally with streaks of anhydrite at the top and with streaks of shale at the bottom. The core analysis of the Middle Marrat formation shows that dolomitization plays a major role for matrix-porosity enhancement, which is responsible for good reservoir character. As part of an exploration strategy, a new structure in northwestern Kuwait was drilled to deep Jurassic formations. This new structure is a northwest/southeast- trending doubly plunging anticline and is intersected by a number of longitudinal- and transverse-fault sets. Earlier, a significant discovery of light oil and gas from the new deeper structure was made from an unsuccessful Paleozoic well through re-entry. After the discovery of light oil and gas, three more exploration/delineation wells were drilled to confirm the reserves potential of this structure. Some of the wells are located near the crest of the structure, whereas some are 1.5 km from the crest (Fig. 1).

Extending Well-Productivity Life in a High-Pressure/ High-Temperature Tight Gas/Condensate Carbonate Reservoir by Acid Refracturing

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 142444, ’Extending Well- Productivity Life by Acid Refracturing in a High-Pressure/High-Temperature Tight Gas/Condensate Carbonate Reservoir in a Jurassic Formation of Northern Kuwait,’ by Mir Kabir, SPE, Qasem Dashti, SPE, J.R. Singh, SPE, and San Prasad Pradhan, SPE, KOC, and Ikhsan Nugraha, SPE, Liu Hai, and Hamed Al-Ghadhban, Schlumberger, prepared for the 2011 SPE Middle East Unconventional Gas Conference and Exhibition, Muscat, Oman, 31 January- 2 February. The paper has not been peer reviewed. Productivity of the subject well, which had been on production for 3 years after an initial acid-fracture treatment with almost 90% of its cumulative production occurring during the last year, was declining faster than expected. Review of the well’s performance indicated two possible causes: reservoir-pressure decline and reduced fracture-length/conductivity ratio. Acid refracturing was chosen to restore the well’s productivity by extending the fracture half-length to enhance fracture conductivity. The results showed that the productivity index (PI) increased by a factor of 2.5 during the post-treatment production test. However, after 3 months of production, this declined to 1.2. Introduction Several major fields in north-ern Kuwait, including Sabriyah, Raudathain, Umm Niqqa, Bahra, and Dhabi, produce gas from the Jurassic Middle Marrat formation, an important carbonate reservoir. Typical depth is 14,000 ft, reservoir pressure ranges from 10,000 to 12,000 psi, and the bottomhole static temperature averages 275°F. The Middle Marrat formation is mostly limestone (calcite>90%) with minor dolomitization. Only traces of clay and silt are found. High heterogeneity, both laterally and vertically, has been reported. Core permeability matches log permeability, and the high permeability value obtained from the core could be the result of induced fractures. Pressure-transient analyses of some wells showed dual-porosity behavior with low matrix permeability. Generally, the absolute matrix permeability is less than 1 md. The production mechanism is a combination of matrix and natural-fracture permeability. Acid Refracturing In regions where acid fracturing is common, there are questions that arise. When is the best time to refracture? What is the justification to repeat the treatment? If there is justification, what needs to be done differently in the subsequent treatment? The decision usually relies on the post-treatment production behavior compared with the pretreatment condition.

Update to Late Triassic – Jurassic stratigraphy of Saudi Arabia for the Middle East Geologic Time Scale

ABSTRACT This Note presents a formal update to the Middle East Geologic Time Scale 2008 (ME GTS) for the Late Triassic and Jurassic rock units of Saudi Arabia. It reviews their lithostratigraphic nomenclature, ranks and stage assignments, and proposes names for third-order chrono-sequences as compiled and/or interpreted from the published literature. The review starts with the Late Triassic (Late Norian – Rhaetian) Minjur Sandstone of the Buraydah Group, with the Triassic – Jurassic (TrJ) boundary positioned at its top. The Minjur Sandstone consists of two units, here ranked as members, which are interpreted as the Late Norian – ?Early Rhaetian Lower Minjur Sequence (Lower Minjur Member) and ?Late Norian – Rhaetian Upper Minjur Sequence (Upper Minjur Member). The Early Jurassic Unconformity - Hiatus (Hettangian, Sinemurian and Pliensbachian stages) separates the Minjur Sandstone from the Toarcian Marrat Formation of the Jurassic Shaqra’ Group. The Shaqra’ Group consists of seven formations, from base-up: (1) Marrat Formation consisting of the Lower, Middle and Upper units, here ranked as members. (2) Dhruma Formation consisting of eight units (D1–D5, Wadi ad Dawasir “delta”, D6 and D7). The Dhruma units have been named in the literature as Balum Member (D1 and lower part of D2 units), Dhibi Limestone Member (upper part of D2 unit), Uwaynid Member (D3), Barrah Member (D4), Mishraq Member (D5), ‘Atash and Hisyan members (D7); the D6 and Wadi ad Dawasir “delta” units are not formally named as members. (3) Tuwaiq Mountain Limestone consisting of the Baladiyah (T1 unit), Mysiyah (T2 unit) and Daddiyah (T3 unit) members. (4) Hanifa Formation consisting of the Hawtah and Ulayyah members. (5) Jubaila Limestone with J1 and J2 units. (6) Arab Formation consisting of D to A members. (7) Hith Anhydrite consisting of Main Hith Anhydrite Member and Manifa Reservoir/Member. The seven formations of the Jurassic Shaqra’ Group are interpreted as 11 third-order chrono-sequences: (1) Early Toarcian Marrat Sequence B (Lower and Middle Marrat members) and Mid- to ?Late Toarcian Marrat Sequence A (Upper Marrat Member), the latter containing Arabian Plate maximum flooding surface MFS J10. (2) Bajocian Lower Dhruma Sequence (Balum Member and Dhibi Limestone), formed by the Balum and Dhibi subsequences, the former containing MFS J20. (3) Late Bajocian – Mid-Bathonian Dhruma Sequence B (Uwaynid, Barrah, Mishraq members, and Wadi ad Dawasir “delta” unit), with the Mishraq containing MFS J30. (4) Late Bathonian – early Mid-Callovian Dhruma Sequence A (unit D6, ‘Atash and Hisyan members), with the Hisyan containing MFS J40. (5) Mid- to Late Callovian Tuwaiq Sequence (Tuwaiq Mountain Limestone) containing an undesignated MFS at the base the Daddiyah Member (T3 unit). (6) Early and Mid-Oxfordian Hawtah Sequence (Hawtah Member of Hanifa Formation) containing MFS J50. (7) Late Oxfordian – ?Early Kimmeridgian Ulayyah Sequence (Ulayyah Member of Hanifa Formation) containing MFS J60. (8) Kimmeridgian Jubaila Sequence (Jubaila Limestone) containing MFS J70. (9) Arab-D Sequence (Arab-D Member inclusive of the Arab-D Anhydrite) containing an undesignated MFS in the Arab-D carbonate. (10) ?Kimmeridgian – ?Tithonian Arab-C and B Sequence (Arab C and B members) containing fourth-order MFS J80 and J90 in the lower carbonates of the members; and (11) ?Kimmeridgian – Tithonian Arab-A - Main Hith Sequence (Arab-A Member and Main Hith Anhydrite below the Manifa Reservoir/Member) containing fourth-order MFS J100 in the Arab-A Member. The Tithonian Manifa Member (upper part of the Hith Anhydrite Formation) is interpreted as heralding a transgression (fourth-order MFS J110), which deposited the Late Jurassic – Early Cretaceous Sulaiy Formation of the Thamama Group. The Jurassic – Cretaceous (JK) boundary is placed in the Sulaiy Formation by stratigraphic position. Based on the Geologic Time Scale GTS 2004 and Arabian Orbital Stratigraphy (AROS), the ages in million years before present of the sequence boundaries and maximum flooding surfaces are estimated for these chrono-sequences.

Les Brachiopodes du Lias-Dogger: Paléontologie et biostratigraphie

Field-trips in the central part of Saudi Arabia, withsearches of B.R.G.M. so as to realize geological maps, have made possible the establishment of the lithostratigraphy and the chronostratigraphy of the Jurassic. Numerous well situated fossils (cephalopods, brachiopods, bivalves, gastropods, echinids, crinoids) have been collected in many sections. Beds between the Marrat Formation (Toarcian) and the top of the Tuwaiq Formation (Callovian) have provided more than 4300 brachiopod shells. The object of the present work is: 1) to makeknown the morphology of the main brachiopod species which follow one another in the platform environments (middle and proximal infralittoral biotopes) during the Lower and Middle Jurassic. The stratigraphical distributions of the species are justified by the representation of shells from different levels; 2) to establish a biostratigraphical scale of brachiopods. The main results are as follow: - an alone bed with brachiopods has been found inthe Toarcian, at the bottom of the Middle Marrat Formation (Lower Toarcian, Serpentinus Zone); - the brachiopod faunas of the Lower part of the Dhruma Formation (Bajocian except the Garantiana and Parkinsoni Zones) are very different to the above faunas. Some species show an european mark ( Monsardithyris ventricosa, Sphaeroidothyris cf. sphaeroidalis), other species have been described already on the south side of the Mesogea, mainly in Morocco ( Burmirhynchia termeirae, Tubithyris chouberti, Rugitela praebullata) or in Egypt and in Sinaï Peninsula ( Eudesia magharensis). The fig. 8 shows for the first time a succession of five species associations; the main species of which were previously referred to a total age Bajocian-Bathonian and even Callovian. Turning to discontinuities, no fauna known elsewhere from the single Middle Bathonian has been observed. A gap of the Middle Bathonian and perhaps of a part of the Upper Bathonian can be looked at the top of the Barrah Member of the Dhruma Formation otherwise characterized by generalized hard-grounds. Effectively, there are only two polymorphic species with a large stratigraphical distribution ( Daghanirhynchia subversabilis and Cererithyris bihinensis) in this Barrah Member (beds with Microphalites and moreover the brachiopod associations 3 and 4 which enclosed it are entirely different. This fact suggests rather important variations of the palaeoenvironments. The most noticeable of these variations preferentially succeed to the sedimentation stoppings